1. Field of the Invention
The present invention is related, in general, to methods of wireless communication.
2. Description of the Related Art
An innovative aspect in third generation (3G) wireless communication systems is the offering of high-speed packet data services to end users. The Universal Mobile Telecommunications System (UMTS) may provide improved voice capacity as compared to earlier systems. However, the bulk of the innovation may lie in the ability of UMTS to provide a “fat pipe”, or a shared channel, where packet data users are served with time-sharing or scheduling based on knowledge of their instantaneous channel quality. In UMTS, this channel is generally referred to as a high-speed downlink packet access (HSPDA) channel.
A wireless channel is inherently different from a wire line channel, in that a wireless channel is dynamic and has a time-varying capacity to carry information. Higher layer protocols such as transmission control protocol (TCP) have been designed based on a premise that the links are constant capacity links and any failure to get a packet across is most likely due to congestion in the network elements. This assumption is not true for transmission over a single wireless link. Diversity, i.e., when the same signal arrives at the destination via multi independent paths, plays a role in wireless channel reliability. The diversity paths may be obtained in space (by multiple transmit or receive antennas) and/or in frequency (by resolving multiple propagation paths at different delays), and/or in time (by transmitting the same information over multiple blocks of time).
Spatial diversity requires the setting up and utilizing of additional antenna elements in the infrastructure and/or the mobile station, and multi-path diversity is a function of the propagation environment. On the other hand, utilization of time diversity may enable reliable packet data transmission in the packet data network when the last link is a wireless channel. This type of transmission may be achieved via what is referred to as Hybrid Automatic Repeat reQuest (HARQ). In a HARQ transmission, the same information is re-transmitted with a possibly different code, if one or more of the previous transmissions have failed. However, stabilizing effects of HARQ for improving link reliability may come at the cost of increased delay.
To enable a HARQ transmission, a reverse link channel (mobile to base station) should be available, which can signal, to the base station transmitter, if the particular transmission has been successful or not. This uplink channel should be designed to carry a signaling message containing a positive acknowledgement (ACK), or a negative acknowledgement (NACK), and/or not transmit anything (known as discontinuous transmission (DTX) frame, a frame with no data in it) when there is no packet to acknowledge. This signaling channel is known as the high-speed dedicated physical control channel (HS-DPCCH) in UMTS. The downlink (base station to mobile) capacity of a data channel such as HSDPA depends on the ability of the uplink receiver at the base station, to correctly detect these ACK/NACK/DTX signals. When a particular user is not scheduled by the base station (also known as the NodeB) transmitter, the ACK/NACK detector at the NodeB can know that and may expect a DTX (silence).
On the other hand, when the user has been scheduled, all three states (ACK/NACK/DTX) are possible. Although an ACK or NACK may be expected, a DTX may also be possible if the user did not decode that the downlink HSDPA transmission was intended for it. The downlink channel that carries the information as to whether a user is scheduled is known as a high-speed shared control channel (HS-SCCH). Thus, if the mobile (also known as a user equipment (UE)) misses decoding the HS-SCCH when it was actually being transmitted to it, the UE will not transmit anything (no message) on the HS-DPCCH signaling channel, even though the NodeB transmitter is expecting either an ACK bit or a NACK bit in the signaling message carried on the HS-DPCCH.